Electron guns



Mayh26, 1970 J. D. M CANN 3,514,664

ELECTRON GUNS Filed Oct. 28. 1968 I .4 Sheets-Sheet 1 J. D. M CANN ELECTRON GUNS May 26, 1970 .4 Sheets-Sheet 2 Filed Oct. 28, 1968 J. p. MOCANN May 26, 1970 ELECTRON GUNS .4 Sheets-Sheet 5 Filed Opt. 28, 1968 J. D. M CANN ELECTRON GUNS May 26, 1970 .4 Sheets-Sheet 4 Filed Oct. 28. 1968 3,514,664 ELECTRON GUNS John David McCann, Abingdon, England, assignor to United Kingdom Atomic Energy Authority, London, England Filed Oct. 28, 1968, Ser. No. 771,045 Claims priority, application Great Britain, Oct. 31, 1967, 49,528/ 67 Int. Cl. H01v 29/56 U.S. Cl. 315-31 14 Claims ABSTRACT 61 THE DISCLOSURE An electron gun comprising a tubular anode having an inner electrode assembly disposed therein. The space defined, by the anode is evacuable and the inner electrode assembly includes an elongated electron emitter and a .first electrode positioned between the emitter and an electron permeable portion of the anode. A greater potential difference is applied between the first electrode and the anode than between the first electrode and the emitter. The first electrode serves to focus, direct and accelerate the electrons emitted by the emitter and also serves to prevent the electric field between the first electrode and the anode from penetrating the space between the first electrode and the emitter.

This invention relates to electron guns and is particularly concerned with guns adapted to emit a linear or sheet like electron beam through a window on to objects to be irradiated.

According to the present invention an electron gun comprises a tubular anode having an electron permeable portion, said anode defining an evacuable space, an electrode assembly in said space, said electrode assembly comprising an elongated electron emitter and a first electrode positioned between said anode and said emitter, and means to maintain a greater potential difference between the anode and said first electrode than between said first electrode and said emitter, said first electrode serving to focus and direct the electrons emitted by said emitter towards, the anode and serving to prevent the electric field between said first electrode and said anode from penetrating the space between said first electrode and said emitter.

Preferably the first electrode includes a mesh portion through Which the electrons emitted by said emitter pass. The first electrode is preferably tubular and may be right circular cylindrical or may be of any desired crosssectional. shape. The emitter may be heated by a DC. voltage or by an A.C. voltage.

Additional electrodes may be provided between the first electrode and the emitter for the purposes hereinafter described.

In order to enable the invention to be more readily understood reference is directed to the accompanying drawings wherein:

FIG. 1 is transverse cross-section of a linear emitter type electron gun,

FIG. 2 is a view similar to FIG. 1 but to an enlarged scale illustrating a modification of the first or inner electrode assembly of FIG. 1,

FIG. 3 is a view also similar to FIG. 1 and to an enlarged scale of a double beam gun,

FIG. 4 is a view similar to FIG. 1 but to an enlarged scale illustrating a further modification of the first or inner electrode assembly of FIG. 1,

FIG. 5 is a view of the electrodes of the first or inner electrode assembly of the gun of FIG. 4 on an enlarged scale,

7 nited States Patent 0 3,514,664 Patented May 26, 1970 FIGS. 6 and 7 are views similar to FIG. 1 but to en larged scales illustrating two further modifications of the first or inner electrode assembly of FIG. 1,

FIG. 8 is a view similar to FIG. 1 illustrating a double beam gun, and

FIG. 9 is a view similar to FIG. 1 but to an enlarged scale illustrating an electron gun capable of producing three or more electron beams.

Referring to FIG. 1 a long straight filament 1 is supported by hooks 2 within a first slotted metal tube. The tube 3 is mounted within a second slotted metal tube 5 the 'slot in tube 5 beingaligned with the slot in tube"? and fitted with a wire mesh 7 and plates 8 defining the edge of the slots. A third outer slotted tube 9 has its slot aligned with the slots of tubes 3 and 5 surrounds tubes 3 and 5 and forms a casing. The slot in the outer tube 9 is provided with a frame 11 for an aluminium foil window 12 and mounted inside the frame each side of the window 12 is an anode plate 13. Other suitable materials for the window are, for example, aluminium alloy or titanium. The tube 9 is evacuated to a pressure, for example, in the region of 10- torr.

The tubes 3, 5 and 9 are as long as desired and are mounted in suitable end insulators which locate the tubes and enable the necessary high voltages to be maintained between them.

The outer tube 9 is earthed, the tube 5 held at a high potential (25-300 kv.) negative to earth, the tube 3, together with the filament 1, at a comparatively low potential (2004000 v.) negative with respect to the tube 5 and the filament 1, has an electrical heating current applied to it. It will be appreciated that the slotted tubes 3 and 5 also function as focussing electrodes for the electrons emitted by the filament 1, and the magnified electron image of the emitter respectively.

The first section is thus operated under low voltage gradients, typically about 500 volts per cm. Under these conditions, the electron optics can be designed to give a 1 cm. wide electron image of the emitter from a 0.025 cm. diameter filament. Also under these low gradients, space charge limitations of current occurs and electron density in the beam is more dependent on first section voltage than filament temperature. This feature is most desirable in a linear type gun where temperature variations along the length of the filament can occur because of changes in cross-section, effects of supports or surface conditions. The magnified electron image is also important in a linear gun since the emitting surface of a long filament can be quite large and any effect which permits a smaller diameter filament, increases the thermal efficiency of the gun assembly.

The electron beam produced in the first section is admitted to the second section by means of the metallic mesh 7 separating the low gradient from the high gradient sections. Typically the wire mesh 7 would have about 20 wires per inch of 0.010" diameter and the wires may be inclined at an angle of about 20 to the centre line of the beam so as to prevent striations in the final beam coming from shadows of the mesh.

The voltage gradient in the second section is typically about 30 kv. per cm. and the provision of the mesh 7 ensures that the field lines in the second section are almost that of coaxial cylinders, only modified by the anode plates 13 and slight change in radius at the position of the mesh 7. The mesh 7 fulfils a further important role of preventing the electric field of the second section from penetrating the first section and thus affecting the electric field of the first section.

The very simple second section optics give about 2:1 further magnification to the diverging beam injected at the mesh 7 when the ratio of inner tube to outer tube is 22711. This ratio also gives the best high voltage performance with the coaxial tu'be configuration. Since the injection energy is quite high the simple optics maintain their performance over a wide range of second section voltages. There is very little change in output beam width between second section voltages of 25 kv. and 300 kv. The output beam width is also relatively unaffected by altering the first section voltage between 200 and 2000 v. Outside the range of voltages quoted, the gun still performs well but some of the operating flexibility is lost.

In the modification shown in FIG. 2 the first slotted tube designated 3a is constructed of magnetic material such as mild steel and a filament feed wire 14 (suitably insulated) is passed along the outside of the tube 3a. This wire 14, together with the filament, forms a one-turn induction loop around the tube 3a. The transverse field 15 produced deflects the low energy electrons in the first acceleration stage longitudinally by an angle sufiicient to eliminate shadows from the filament supports 2.

FIG. 3 shows a two stage gun to give more than one output beam from the same vacuum tube. This provision of multiple beams displaced at angles to each other offers great scope for the irradiation of irregular objects as well as giving greater 'beam output per tube whilst keeping the exit window loading down to reasonable levels.

On guns greater than for example 12 inches in length using filaments of for example 2 volts per inch and having an A.C. voltage in the region of 140 volts applied across its ends with a voltage gradient of 500 volts between the tube 3 and the tube 5, non-linearity of the output beam along the gun may occur. The non-linearity of the output beam results from fluctuations of the filament voltage affecting the electron optics of the gun, and can be eliminated by replacing tube 3 of FIG. 1 or tube 3a of FIGURE 2 with two focussing electrodes 16 as shown in FIGS. 4 and 5.

'Referring to FIGS. 4 and 5 it will be seen that the filament 1 is supported from a support plate 17, which is mounted in insulators (not shown) at its ends. Insulated filament supports 18 are provided in the support plate 17. The focussing electrodes 16 are attached to, and supported from, the support plate by fixing screws 19'. Insulators 20 are provided between the focussing electrodes 16 and the support plate 17. The focussing electrodes 16 are supplied with an A.C. voltage proportional to the A.C. voltage applied to the filament 1, such that at any point along their length they have substantially the same instantaneous A.C. voltage applied to them as the instantaneous A.C. voltage at the corresponding point along the length of the filament. Ideally, the electrodes 16 should have a smooth continuous voltage variation along their length in a similar manner to the filament 1. Each focussing electrode 16 may be constructed from a thin metal plate having a high resistivity to keep power consumption within reasonable practical limits. However segmentation of each electrode 16 into a plurality of plates 21 (FIG. 5) for example of 3 to 4 inches long, gives good results when the power supply is arranged to divide the total A.C. voltage applied to the electrodes 16 such that the instantaneous voltage applied to each plate 21 matches the instantaneous A.C. voltage of the region of the filament adjacent the respective plate. To achieve this condition the plates 21 are arranged in pairs interconnected by leads 22 with one plate 21 disposed on one side of filament 1, and the other plate 21 disposed on the other side of the filament 1. The pairs of plates 21 are positioned along the length of the filament and are connected to adjacent pairs of plates 21 by means of resistors 23 which are connected in series. The resistors 23 are connected to the mid-point of one plate 21 of each pair of plates 21. The pair of plates 21, adjacent each end of the filament 1, are either connected to the end terminals 24 and 25 of the filament 1, or to independent terminals (not shown) by means of a resistor 26 which has a resistance half that of the resistors 23. The resistance of the resistors 23 annd 26 is chosen such that the instantaneous A.C. voltage at the mid-point along each pair of plates 21 matches the instantaneous A.C. voltage of the filament 1, adjacent the mid-point of the respective pair of plates 21. Hence the maximum difference in the instantaneous A.C. voltages between the filament and the electrodes 16 occurs at the ends of each plate 21 and is for example in the order of 3 to 4 volts R.M.S.

The power supply to the filament is applied across the terminals 24 and 25. Substantially the same D.C. negative bias with respect to the tube 5 may be applied to the filament 1, the electrodes 16 and the support plate 17 Alternatively, a potential dilference may be maintained between the filament 1 and the electrodes 16 by making the electrodes 16 positive with respect to the filament, whilst maintaining the electrodes negative with respect to the tube 5. Variations of the potential difference between the filament 1, and the electrodes 16 could be used to vary the intensity and shape of the electron beam emerging from the first section of the gun.

By operating the gun with substantially no D.C. potential difference between the filament 1 and the electrodes 16 but with comparable A.C. voltages applied to the filament 1, and the electrodes 16 the electrons leaving the filament 1 are not influenced by the A.C. voltage applied to the filament 1, and hence a more uniform beam is projected into the second electron accelerating stage of the gun.

Referring to FIG. 6 there is shown a modified first section of an electron gun similar to that shown in FIG. 1. The first section of the gun uses a filament 1 arranged to operate with about volts A.C. applied across its ends. The section is modified in that an electrode 27 is positioned between the filament 1 and the wire mesh window 7. In operation of the gun the filament 1 together with the filament support plate 17 has a negative bias of, for example, up to l kilovolt applied to them and the electrode 27 is maintained at for example between 50 to 200 volts D.C. positive with respect to the filament 1. As the A.C. voltage applied across the ends of the filament 1 is of a similar order to the DC. potential difference between the filament 1 and the electrode 27 it must be compensated for. The electrode 27 may be made from a metal mesh of high resistance extending the length of the filament 1, to form a grid to enable a smooth voltage gradient to be obtained along the electrode 27 or alternatively may be made from a number of electrically interconnected metal mesh members and employ a similar electrical circuit to that for dividing the voltage along the length ofthe electrodes 16 of FIG. 5. The electrode 27 has applied to it an A.C. voltage proportional to and in phase with, the A.C. voltage applied to the filament 1. In this way the electrode 27 would elfectively only have the negative bias to accelerate the electrons emitted by the filament 1. Although the electrode 27 will give an almost uniform emission along the length of the filament, and with respect to time, the net electron energy from this stage of acceleration will contain varying energy components. By subjecting the electrons to a further stage of acceleration before injecting the electrons into the high voltage section of the gun this can be minimized if not eliminated. The latter mentioned further stage of acceleration is achieved by maintaining the tube 5 in the order of, for example, up to 1000 volts positive with respect to the filament 1 whilst still maintaining the tube 5 negative with respect to the tube 9. The electron beam is then projected into the high voltage gradient section of the gun that exists between the tubes 5 and 9 where the main acceleration of the beam is carried out.

The main control of the intensity of the beam current would be by varying the DC. bias applied to the mesh electrode 27.

Ifl desired the electrode 27 of FIG. 6 may be incorporated into the electron gun of FIG. 4 in which case the electrodes 16 serve to produce a sharper electron image of the filament 1 at the exit from the first section of the gun.

A further modification to the electron gun of FIG. 6 is shown in FIG. 7. Referring to FIG. 7 it will be seen that the mesh electrode 27 has been replaced by two electrodes 28. Each electrode 28 may be made from one member which extends along the length of the filament 1, or may be made from a number of electrically interconnected plates arranged in a similar manner to the electrode 16 of the gun shown in FIG. 5. The filament 1 is supported from the support plate 17 by means of the insulated filament support 18. The electrodes 28 are fastened to.the support plate 18 by screws 29 and are insulated from the support plate 18 by insulators 30. The electrodes 28 extend along the length of the filament 1 andare supplied with a D.C. bias together with an A.C. voltage in a similar manner to that applied to the electrode 27 of FIG. -6. The electrodes 28 perform a similar function to the electrode 27 of FIG. 6 The lower edges of the electrodes 28 are shaped so as to define a longitudinal tunnel of tapered cross-ection with the narrow opening at the top of the tunnel adjacent the filament 1.\ The tunnel serves to focus and direct the electrons emitted by the filament towards the mesh 7 in the tube 5.

It is to be understood that the electrodes 28 of FIG. 7 may be incorporated in the electron gun of FIG. 4 in which case the electrodes 16 are positioned between the filament 1 and the electrodes 28.

FIG. 8 shows a two-stage gun similar to that of FIG. 6 to give more than one output beam from the same vacuum tube 9. Two aluminium foil windows 12 are provided in the outer tube 9 at diametrically opposite positions. The filament 1 is supported from the support plate 17 by insulated filament supports 18. An additional plate 31 substantially similar to the support plate 17 is provided within the tube 5 to maintain symmetry of the gun. Two mesh windows 7 are provided in the tube 5 and arranged to coincide with the windows 12 in the tube ,9. A mesh electrode 27 similar to that of FIG. 4 is positioned between the filament 1 and each mesh window 7. ,The operation of the gun of FIG. 8 is substantially the same as that of the gun of FIG. 4 except that the plate 31 of FIG. 8 is maintained at a D.C. potential. similar to that applied to the support plate 17 and two electron beams 180 apart are produced.

Referring to FIG. 9 there is shown an electron gun for producing two or more electron beams from one vacuum tube. The first low voltage gradient section of the. gun is provided with a number of electrodes 32 and 33,.which serve to split the stream of electrons emitted by the filament 1 into three separate beams. The filament 1, is. supported from a support plate 17 by insulated filament supports 18. The tube 5 is provided with three longitudinally extending slots fitted with a wire mesh 7. The mesh 7 is clamped to the tube 5 by plates 34. The electrodes 32 .land 33 are in the form of metal plates which extend longitudinally with respect to the filament 1 and which are attached along one edge to the plates 34. Each electrode 32, 33 is positioned in a radial plane with respect to the filament 1. The free edges of the electrodes 32 nearest to the filament 1, are closer to the filament 1 than the corresponding free edges of the electrodes 33. The width ofthe electrodes 32 and 33 measured in a radial direction with respect to the filament 1, is chosen to giveequal voltage gradients around the cathode when avoltage is applied to the electrodes 32 and 33. The tube 9 is provided with slots in which metal foil windows I 12 are provided. The slots in the tube 9 are aligned with with respect to the tube 9 which is earthed. A negative D.C. voltage is applied to the filament 1 and the support plate 17 to maintain them at a negative potential with respect to the tube 5 and an electrical heating current is applied across the ends of the filament 1. A gun approximately 15 cms. long constructed according to FIG. 9 produced three substantially equal and uniform beams through the meshes 7. The displacement angle between the beams was set for 30 but it is to be understood that the angle of displacement between the beams is arbitrary. The free edges of the electrodes 32 were positioned at 9 mm. from the filament 1, whilst the free edges of the electrodes 33 were positioned at 12 to 13 mm. fromrthe filament 1..It, is to be understood that the gun may be of any desired length, and the electrodes 32 and 33 may be constructed from a number of plates electrically interconnected in a similar manner to the plates 21 of FIG. 5 so that an A.C. voltage may be applied to the plates forming the electrodes 32 and 33 together with the D.C. voltage applied to the electrodes 32 and 33 by way of the tube 5.

It will be appreciated that the gun assembly may be rotated about its longitudinal axis such that the electron beam may radiate at any angle other than normal to the sample, i.e. when irradiating a sample the beams may be arranged to irradiate the approaching sides of the sample as well as the top surface.

I claim:

1. An electron gun comprising a tubular anode having an electron permeable portion, the walls of said anode defining a closed chamber therebetween and said chamber being at least partially evacuated, an electrode assembly in said chamber, said electrode assembly comprising an elongated electron emitter extending in a direction substantially parallel to the longitudinal axis of said tubular anode, and a first electrode, at least part of which is made of a metallic mesh, positioned between said anode and said emitter, and means for maintaining a greater potential difference between the anode and said first electrode than between said first electrode and said emitter, said first electrode serving to focus and direct the electrons emitted by said emitter towards the anode and serving to prevent the electric field between said first electrode and said anode from penetrating the space between said first electrode and said emitter.

2. An electron gun according to claim 1 wherein said first electrode is tubular.

3. An electron gun according to claim 1 wherein a second electrode is provided between said emitter and said first electrode.

4. An electron gun according to claim 3 wherein a third electrode is provided adjacent said emitter, said third electrode being maintained at substantially the trons emitted by said emitter.

5. An electron gun according to claim 1 wherein an A.C. voltage is applied across the ends of the emitter to heat said emitter together with a negative potential with respect to said first electrode to accelerate the electrons emitted by said emitter.

6. An electron gun according to claim 4 wherein a fourth electrode is provided adjacent said emitter, said fourth electrode having applied thereto an A.C. voltage proportional to the A.C. voltage applied across the ends of said emitter, together with a negative potential with respect to said first electrode such that the instantaneous A.C. voltage at any given region along said fourth electrode is substantially the same as the instantaneous A.C. voltage at a corresponding region along said emitter.

7. An electron gun according to claim 1 wherein at least one longitudinally extending electrode is attached to said first electrode and projects inwardly towards said emitter to split the stream of electrons leaving said emitter into at least two beams.

8. An electron gun according to claim 7 wherein an A.C. voltage is applied to the emitter to heat said emitter, and an A.C. voltage proportional to that applied to the emitter is applied to said at least one electrode attachcd to said first electrode such that the instantaneous voltage at any given region along said at least one electrode is substantially the same as the instantaneous A.C. voltage at a corresponding region along said emitter.

9. An electron gun according to claim 3 wherein an A.C. voltage is applied to said emitter to heat said emitter and an A.C. voltage proportional to the A.C. voltage applied to said emitter is applied to said second electrode, together with a DC. negative potential with respect to said first electrode such that the instantaneous A.C. volt age at any given region along said second electrode is substantially the same as the instantaneous A.C. voltage at a corresponding region along said emitter.

10. An electron gun according to claim 3 wherein the second electrode is made of metallic mesh.

11. An electron gun according to claim 3 wherein the second electrode is made from a number of electrically interconnected metallic mesh members.

12. An electron gun according to claim 6 wherein said fourth electrode is made from a number of electrically interconnected plates.

13. An electron gun according to claim 4 wherein said third electrode is made of a magnetic material and a conductor extending parallel to said emitter is provided and arranged such that the said conductor together with said emitter form a one turn induction loop whereby the electrons emitted by said emitter are magnetically deflected longitudinally.

14. An electron gun according to claim 4, wherein said third electrode comprises a number of electrically interconnected plates.

References Cited UNITED STATES PATENTS 2,475,644 7/ 1949 Soller. 2,496,361 2/1950 Sziklai. 2,074,829 3/1937 Case. 2,190,735 2/ 1940 Rust.

RODNEY D. BENNETT, JR., Primary Examiner J. G. BAXTER, Assistant Examiner US. Cl. X.R. 313-82 

